The J, K and N-class was a class of 24 destroyers of the Royal Navy launched in 1938. They were a return to a smaller vessel, with a heavier torpedo armament, after the Tribal-class that emphasised guns over torpedoes. The ships were built in three flotillas or groups, each consisting of eight ships with names beginning with "J", "K" and "N". The flag superior of the pennant numbers changed from "F" to "G" in 1940.

The ships were modified throughout their war-time service, particularly their anti-aircraft guns; they were also fitted with radar.

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The design was intended as a smaller follow-on from the preceding Tribal-class, and incorporated one radical new idea that was a departure from all previous Royal Navy destroyer designs. That was the adoption of a two boiler room layout. This reduced hull length and allowed for a single funnel, both reducing the profile and increasing the arcs of fire of the light anti-aircraft (A/A) weapons. However, this also increased vulnerability, as there were now two consecutive large compartments with the resultant risk of a single well placed hit flooding both and resulting in a total loss of boiler power. This illustrates somewhat the Admiralty's attitude to the expendable nature of destroyers.[citation needed] This is of course, as opposed to the 3-boiler-room layout that was used starting with the F-class in the early 1930s. Early ships also tended to use twin boiler rooms, which are still a great improvement over a single boiler room. In any case, destroyers are lightly armored and fast vessels meant to survive by avoiding being hit at all. The odds of a single hit striking just the right spot to disable both boiler rooms simultaneously were considered remote enough to be worth risking in exchange for the benefits given by a two-room layout. During Prewar trials "...On a light displacement Jackal attained 37.492 knots, on the Arran mile in 60 fathoms, 34.37 knots deep. Jupiter in 75 fathoms made 33.835 knots light, 33.045 knots deep displacement."[2]

A significant advance in construction techniques was developed by naval architect Albert Percy Cole. Instead of going for transverse frame sections which were unnecessarily strong, but held together by weak longitudinals, Cole went for extra strong longitudinals and weaker transverse frames.[3]

Another advancement was changes to the bow design. The bow form was also modified from that of the preceding Tribal-class design; the clipper bow replaced by a straight stem with increased sheer. This change was not a success and as a consequence these ships were very wet forwards. This shortcoming was rectified from the S-class onwards by returning to the earlier form.

Despite the vulnerability of the boiler layout, the design was to prove compact, strong and very successful, forming the basis of all Royal Navy destroyer construction from the O-class up to the last of the C-class of 1943-1945.

The armament was based on that of the Tribals, but replaced one twin QF 4.7 in (120 mm) Mark XII (L/45) gun on mounting CP Mk.XIX with an additional bank of torpedo tubes. These mountings were capable of 40° elevation and 340° of training. Curiously, 'X' mounting was positioned such that the blind 20° arc was across the stern, rather than the more logical forward position where fire was obscured by the bridge and masts anyway. This meant that they were unable to fire dead astern. With the tubes now 'pentad', a heavy load of 10 Mk.IX torpedoes could be carried. A/A armament remained the same, consisting of a quadruple QF 2 pdr gun Mark VIII on a Mk.VII mounting and a pair of quadruple 0.5 in Vickers machine guns. Armament was further improved by replacing the quadruple machine guns with 20mm Oerlikons.[4] These ships, when completed, had a comparatively heavy close range AA armament.[5] Fire control arrangements also differed from the Tribals, and the dedicated high-angle (H/A) rangefinder director was not fitted, instead only a 12 ft (3.7 m) rangefinder[6] was carried behind the usual Director Control Tower (DCT). In the event, the rangefinder was heavily modified to allow it to control the main armament for A/A fire, and was known as the "3 man modified rangefinder". These ships used the Fuze Keeping Clock HA Fire Control Computer.[7]

The N-class were ordered in 1940 as repeats of the J design, after delays and cost over-runs associated with the larger and more complicated L and M-class. The only design change was to locate the 'X' 4.7-in mounting in the more logical position with the 20° training blindspot forward. While building, the same early wartime modifications as the Js and Ks were applied, with a pair of twin power-operated 0.5 in machine gun turrets briefly carried on the quarterdeck before being replaced by single 20 mm Oerlikons.

Church service aboard HMS Javelin, August 1940. Note the 4-inch QF Mk V anti-aircraft gun at upper left and the depth charges at bottom right

In 1940 and 1941, to improve the anti-aircraft capabilities, all ships had their aft torpedo tubes removed and replaced with a single 4 inch gun QF Mark V on a HA Mark III mounting. The relatively ineffective multiple 0.5-inch (13 mm) machine guns were replaced with a single 20 mm Oerlikon, with a further pair added abreast the searchlight platform amidships. The high-speed destroyer mine sweeps were replaced with a rack and two throwers for 45 depth charges and a Type 286 Radar air warning was added at the masthead alongside Type 285 fire control on the H/A rangefinder-director.

In 1942 the 4 in gun was removed and the torpedoes returned to all surviving vessels. The 20 mm Oerlikons were replaced with twin mountings (except those on the quarterdeck) and a Type 291 Radar replaced the Type 286. Jervis, Kelvin, Nerissa and Norman had the searchlight replaced with the "lantern" for centimetric target indication Radar Type 271; Javelin and Kimberley having the lighter Type 272 fitted at the truck of the foremast. Napier, Nizam and Norseman (and later, Norman) had American SG1 Radar fitted at the head of a new lattice foremast, Norman replacing her Type 271 set with a single 40 mm Bofors gun. By the end of the war, the surviving J and K ships carried a lattice mast with a Type 293 Radar target indication at the truck and a Type 291 air warning at the head.

Being amongst the Royal Navy's most modern and powerful destroyers at the outbreak of war, they were extensively committed. As a result, losses were heavy and of 24 ships built, six J, six K and one N-class were lost.

Seriously damaged by the Italian battleship Littorio during the Second Battle of Sirte, 22 March 1942. Bombed by German aircraft while in dry dock in Valletta on 11 April 1942 and written off as a constructive total loss. Expended as a blockship off Malta

^Hodges and Friedman, Destroyer weapons of WW2. Previous to the Tribal-class, RN destroyers carried either two 2 pdr AA guns or twin quadruple .5 in Vickers machine guns. USN destroyers, in the same time frame, usually carried four .5 in Browning machine guns.

^Langtree, Christopher, The Kellys, British J,K and N-class Destroyers of World War two, p36.